1. Field of the Invention
The present invention relates generally to a bipolar transistor, and more particularly to a silicon/silicon-germanium heterojunction bipolar transistor fabrication method capable of reducing resistance of the extrinsic base electrode and increasing a maximum oscillation frequency and cut-off frequency due to its self-aligned structure, using a metallic silicide film as an extrinsic base electrode.
2. Description of the Prior Art
In the fabrication of a conventional heterojunction bipolar transistor, a non-self-align and a self-align technology are adopted.
As for a self-align technology, with reference to an article published in IEDM by a company named NEC in 1990, FIG. 1 shows a schematic cross-sectional view of a SSSB(Super Self-aligned Selectively grown Base) transistor using silicon/silicon--germanium as a base film. The fabrication of the SSSB transistor will be set forth as follows, referring to FIG. 1.
First, an oxide film 4, a P+ polycrystalline silicon 5 and an insulator film 6 are sequentially deposited on an N+ epitaxial layer 3. Thereafter, the insulator film 6 and the P+ polycrystalline silicon film 5 are sequentially etched using an emitter mask.
An etch-back technique is applied to form a side wall insulator spacer 7 on each side wall of layers having the P+ polycrystalline silicon film 5 and the insulator film 6, and then an oxide film 4 is laterally etched to a certain extent using a wet-etching technique.
A silicon/silicon-germanium film 8 serving as a base film is formed in the etched space using a selective epitaxial layer growth method and an additional side wall insulator spacer 9 is provided on each of the side wall insulator spacers 7 using an etch-back technique. N+ polycrystalline silicon 10 serving as an emitter is selectively grown on the exposed silicon/silicon-germanium film, thereby completing the SSSB transistor fabrication using silicon/silicon-germanium film as a base film.
However, the fabrication method of the above-described conventional silicon/silicon-germanium bipolar transistor has a disadvantage, in that since a wet-etching technique is applied to the oxide film 4, it is difficult to control etch-stop line thereto, whereby parasitic capacitance between the base and the collector cannot be precisely controlled. Also, the above method has an additional disadvantage, in that, to grow the silicon/silicon-germanium film 8 as a base film, a thickness has to be precisely adjusted. Further, because the P+ polycrystalline silicon 5 is employed as the extrinsic base electrode, resistance is much more increased than when using metallic silicide.
Specifically, when a non-self-aligned silicon/silicon-germanium geterojunction bipolar trasistor is formed by using a polycrystalline silicon as an extrinsic base electrode in accordance with the conventional method, a pattern formation can be easily achieved, so that the device performance becomes uniform, yet, resistance of an extrinsic base electrode increases, parasitic capacitances become large and even when a metallic silicide is employed as an extrinsic base electrode, an interface between a metallic silicide film and a silicon/silicon-germanium can lead to surface agglomeration occurring due to heat treatment, thereby making its application to an integrated circuit inappropriate.